1. Field of the Invention
The present invention relates generally to the field of microscopic inspection of membranes, and more particularly to compositions and methods to characterize (identify, locate and measure) microscopic defects in membranes.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98
Membranes are generally useful to separate components of a mixture by selectively allowing transit of certain component(s) and not others. Membranes are widely used in commercial and industrial applications such as water purification via filtration or reverse osmosis, food and drug processing, and energy-related applications like battery and fuel cell technology.
Generally, a membrane is used to discriminately allow passage of liquid and filter particulate. In some cases, the membrane's specifications are geared to allow passage of some particulate dissolved in a liquid while blocking passage of other particles. The most common particulate membranes filtered in commercial applications are selected macromolecules, bacterial pathogens, viruses, salts, colloids, and other impurities.
Structurally, membranes are barriers which serve the function of blocking or retaining contaminants and allowing passage of less contaminated, or non-contaminated fluid. Accordingly, and in light of the potentially devastating consequences to large populations which could result from failure of membranes used in large scale water purification applications, it is essential to inspect regularly the blocking properties and thus the integrity of membranes.
The prior art discloses multiple and, in some cases, costly methods of membrane inspection and identification of membrane defects. Bubble point test methodology is used for membrane defect identification in the broadest range of defect sizes. That type of testing is very expensive and time consuming. The basic commercial equipment required for bubble point testing is priced at approximately $10,000 and requires a trained professional to operate, which amounts to approximately $10,000 per test. The existing equipment requires membrane filtration modules to be taken out of service and pressurized to a nominal operating pressure of 600 psi in order to adequately test a filtration system to a 50 nm size range with water as the solvent.
In fact, in an effort to obtain improved approaches for identifying defects in membranes used for water purification, the US Environmental Protection Agency Office of Water has supported development of technology that utilizes nanoparticles made of gold. In the gold-based technology, the gold particles can be detected by use of anodic stripping voltammetry—a method that requires additional costly and complex equipment, and trained technicians. The use of such a complex electrochemical methods is also severely limited in terms of limit of detection (250,000 gold particles particle per mL of liquid). A further limitation of that method for use as a qualitative or quantitative membrane defect characterization tool is the tendency for small gold particles to agglomerate into larger scale particles. In short, implementation of the gold particle process is expected to be very expensive due to the use of a precious metal. That method is further limited by its inability to detect membrane defects in the 15-50 nm range.
In contrast, the method of the present invention utilzes a simple visual detection of fluorescent solutions which require very low (less than parts per billion) detection limits. In fact, the method of this invention can detect a single fluorescent particle. The cost of implementing and using the method of the present invention is relatively low, in part because it does not require using a precious metal or training on complex equipment.
Membranes are also inspected, and their integrity is evaluated using other techniques. Another known way of testing membranes is to apply a pressure differential across the membrane and measure the time decay of the differential. Air, an inert gas or a vacuum can be used to generate the pressure differential. Pressure differential techniques require complex equipment and a considerable investment of time and therefore are very costly. In some cases, the process in which the membrane is being used must be interrupted to perform the evaluation, which adds to the cost and makes the process less efficient. More traditionally, analysis of the filtered fluid provides a qualitative picture of membrane quality.
More recently, Rajagopalan, et al., U.S. Pat. No. 7,011,758 disclosed and claimed a method for testing the integrity of a membrane which comprises placing a magnetically susceptible material in a fluid upstream of the membrane and collecting the material downstream. Rajagopalan's method uses a magnetic field to collect the magnetic material and a sensor to detect it.
The membrane evaluation techniques of the prior art have numerous shortcomings. For the most part, those techniques provide qualitative information only. In other words, and contrary to the method of the present invention, those techniques give an indication that there is a membrane defect, but do not quantify the extent of it. In fact, the prior art does not disclose a single technique which can pinpoint the place on the membrane where a defect may exist or define whether the membrane complies with performance specifications.